Drill bits used for subterranean drilling, such as those used for drilling oil wells and the like, commonly have a steel body that is connected at the bottom of a drill string. An example such drill bits includes roller cone bits, which have a bit body and one or more roller cones rotatably mounted to the bit body. The bit body is typically formed of steel or another high strength material, and includes cutting elements at selected positions about the cones. The cones and cutting elements rotate and engage with the bottom of a hole being drilled to crush, gouge, and scrape rock for drilling the well. One type of roller cone bit, referred to as a “milled tooth” bit, has roughly trapezoidal teeth protruding from the surface of the cone for engaging the rock, and the teeth are formed of the same material as the cone. The principal faces of such milled teeth that engage the rock are usually hardfaced with a layer of material that is designed to resist wear and/or fracture.
The term “hardfaced” is understood in industry to refer to the process of applying a carbide-containing steel material (e.g., hardfacing) to the underlying steel substrate by welding process, as is described in more detail herein. Thus, the terms “hardfaced layer” or “hardfacing” are understood as referring to the layer of hardmetal that is welded onto the underlying steel substrate.
Conventional hardmetal materials used to provide wear resistance to the underlying steel substrate usually comprises pellets or particles of cemented tungsten carbide (WC—Co) and/or cast carbide particles that are embedded or suspended within a steel matrix. The carbide materials are used to impart properties of wear resistance to the steel matrix. Conventional hardmetal materials useful for forming a hardfaced layer on bits may also include iron-based steel and nickel-based high strength alloys to provide one or more certain desired physical properties. In some cases, carbide or other hard particles may be a phase that exhibits relatively high hardness for wear resistance purposes, while steel or other matrix materials act as another phase providing relatively high strength and fracture toughness.
The hardfaced layer is applied to the milled teeth by oxyacetylene or atomic hydrogen welding. The hardfacing process makes use of a welding “rod” or stick that is formed of a tube of mild steel sheet enclosing a filler which is made up of primarily carbide particles. The filler may also include deoxidizer for the steel, flux, and resin binder. The relatively wear resistant filler material is typically applied to the underlying steel tooth surface, and the underlying tooth surface is thus hardfaced, by melting an end of the rod on the face of the tooth. The steel tube melts to weld to the steel tooth and provide the matrix for the carbide particles in the tube.